In a steam turbine the thermal energy from steam supplied to the turbine is converted into mechanical work. Known steam turbines of this kind comprise a high-pressure side steam inlet and a low-pressure side steam outlet. A control device for controlling at least the steam inlet, but usually for controlling additional system components as well, is also provided. A shaft extending through the turbine, what is referred to as the turbine rotor, is driven with the aid of turbine vanes. Coupling the rotor to an electric generator makes a steam turbine system possible, for example the production of electrical energy.
Rotor vanes and guide vanes are typically provided for driving the rotor. The rotor vanes are secured to the rotor and rotate therewith, whereas the guide vanes are usually fixedly arranged on a turbine housing. Alternatively the guide vanes may, for example, be secured to what is known as guide vane carriers. The guide vanes ensure a good flow of steam through the turbine in order to achieve optimally efficient energy conversion. The temperature and the pressure of the steam are reduced in the route between steam inlet and steam outlet during this conversion.
In principle an optimally low pressure of the steam to be let out should be sought for reasons of efficiency. One problem associated with low outlet pressures, however, is what is known as impingement corrosion which leads to high wear of the rotor vanes.
Owing to the saturated steam state being attained in a low-pressure section of the turbine, moisture that has condensed out of the steam can precipitate and form water droplets in the turbine. Water droplets entrained by the flow of steam collide with the rotating rotor vanes with a high level of energy so the vanes are subject to corresponding wear.
As even hardened steel is removed as a result of this effect, high expenditure on manufacturing optimally resistant rotor vanes, for example by way of coatings made of special material, is the upshot in practice.
Apart from the high costs of specially-coated rotor vanes there is often the problem that these rotor vanes allow comparatively low maximum application temperatures, for example only up to about 120° C. While it is quite possible to design steam turbine systems in such a way that during normal operation corresponding maximum temperatures are not exceeded in a low-pressure section of the turbine, the no-load or low-load operation of the steam turbine, which is required at times in practice and at which the temperature is increased in the low-pressure section, for example to about 200 to 250° C. or more, due to the effect of what is referred to as ventilation, is a problem.
During ventilation the steam in the low-pressure section (for example end stage), which has already been extensively expanded and cooled in preceding turbine sections, is heated again by the rotating rotor vanes.
Apart from the fact that this kind of ventilation impairs the energy conversion efficiency in the low-load range, the elevated temperature prevents a plurality of materials being used to manufacture rotor vanes in the low-pressure section which would otherwise be preferred, for example owing to their high specific strength compared with steel. The use of fiber composite vanes (for example CFRP) or other lightweight vanes, whose basic vane material and/or optionally provided coating allows only a lower maximum temperature, should, for example, be considered in this connection.